We migrated the source of truth for what the active draft and published
versions of course and v1 library content are to the
SplitModulestoreCourseIndex Django model. But the contentpruning
code (structures.py) that was developed in tubular and will be moved to
edx-platform is not aware of this newer model, and still only pulls its
source of truth from MongoDB. So we *must* continue to do writes to
MongoDB, or the pruning code will start pruning live versions.
The longer term fix for this is to make the pruning code aware of
SplitModulestoreCourseIndex, which will be easier once it's moved into
edx-platform.
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